The role of suppressor T cells will be further studied. The subset of normal suppressor T cells from SJL mice and other strains will be studied for the presence of different Ia determinants. The interaction of syngeneic T cells from spleens of unprimed animals with T and B cells from spleens of primed animals will be investigated. As unprimed splenic T cells bring about IgE antibody suppression, it will be investigated if these cells interact with IgE producing B cells, IgE specific helper T cells or eventually with a subset of IgE Suppressor T cells which by themselves without interaction with unprimed T cells are unable to bring about IgE antibody suppression. The substances extracted from unprimed T cells will be further characterized, as for molecular size, presence or absence of Ia determinants on this substance, and methods of purification will be used to obtain a better characterized substance of which the chemical properties may be studied.